A percentage of fires each year are caused by electrical branch circuit wiring arcing faults involving currents below the trip level of a conventional circuit breaker or OCPD (over current protection device) as well as below the handle rating of the breaker. Basic overcurrent protection afforded by circuit breakers is designed to prevent I2R heating of the wiring in an electrical distribution system, which is typically caused by circuit overloading due to short circuits and not arcing faults. A true short circuit is a rarity in an electrical system. In fact, it is more accurate to think of electrical faults as being an arc fault with some level of impedance (low current) or with a low impedance (high current). Many electrical faults begin as high impedance breakdowns between the line and neutral conductors or between the line conductor and the ground wire or device components. AFCI (Arc Fault Circuit Interrupter) technology affords protection from conditions that may not necessarily be an immediate threat but could become hazardous if left unattended.
In order to start a fire, three elements must be present: fuel, oxygen (air), and energy to ignite the fuel. Arcing is defined as a luminous discharge of electricity across an insulating medium. The electrical discharge of an arc can reach temperatures of several thousand degrees Celsius. Arcing produces sufficient energy to reach the ignition point of nearby combustible material(s) before a circuit breaker can respond. Arc detection is an enhancement to thermal magnetic overload detection typically used in circuit breakers or OCPD's, which alone usually do not detect and respond to arc faults.
There are different types of arc faults such as those that are known as “A-type” which occur across a break in the line or neutral conductors or at a loose terminal in a branch circuit of a distribution network. The conductors are carrying current to a load derived from the line voltage. The arc could likewise occur as a break in the conductors or at a loose terminal associated with an extension cord deriving power from line voltage, thereby completing the circuit to the load. Since the current through the A-type fault is limited by the impedance of the load itself, i.e., because the fault is in series with the load, an A-type fault is also known as a “series fault.”
“B-type” arc faults are a second arcing condition. In a B-type fault, the arc occurs between two conductors in the branch circuit or extension cords plugged into it, at a site where the insulating media separating the two conductors has been compromised, e.g., by a staple penetrating the middle of an extension cord causing part of the insulation between the conductors to be nullified. The arc may occur across the line and neutral conductors or the line and ground conductors, or in the case of reverse polarity where the line voltage is reverse-polarized, between the neutral and ground conductors. The current through the B-type fault is not limited by the impedance of the load, but rather by the available current from the supply, as established by the impedance of the conductors and terminals between the source of line voltage and the position of the parallel fault, i.e., the conductive members carrying the current. Since B-type faults are effectively across the line, they are also known as “parallel faults.”
A primary problem in AFCI design is identifying an arc fault, such as A-type or B-type arc faults, without falsely identifying normal loads, such as phase controllers such as light dimmers that commonly employ a solid state switching device such as a triac. These devices tend to mimic certain characteristics of arc faults. The user varies the current delay angle of the switching device, which is the particular phase angle on each half cycle of the line voltage when the switching device becomes abruptly conductive. Once the switching device is conductive, the light or other controlled load is electrically connected to the line voltage for the remaining portion of the half line cycle, at which time the instantaneous current through the controlled load is at or near zero and the switching device turns off. The process of the switching device turning on at the current delay angle and turning off the end of the half cycle is repeated for subsequent half cycles. Each time the switching device turns on there is a high rate of change of current through the controlled load. The repetitive abrupt appearance of current is not unlike the behavior of arcing faults, wherein a sufficient instantaneous line voltage is necessary in order for the arc to strike. The current through the arc fault abruptly commences when the arc is struck, producing a high rate of change of load current similar to the switching device turning on at the current delay angle.
There is a need for an arc fault circuit interrupter that improves upon prior art devices that have detected arc faults on the basis of changes in the current which are unable to distinguish between signals produced by arc faults from those produced by phase controllers such as light dimmers. Methods have been devised that try to distinguish the two origins of changes in the current, for example, on the basis of the steepness of the edge of the first derivative of the current, i.e., the current di/dt, and on di/dt pulse amplitude or repetition rate patterns, but an AFCI using these methods is still prone to either a failure to detect and interrupt a true arc fault hazard or prone to false interruption due to phase controllers depending on the chosen sensitivity of the AFCI's detector.